1. Field of the Invention
The present invention relates to a novel photosensitive composition. More specifically it relates to a novel photosensitive composition capable of forming fine patterns by a light of a wave length of 330 to 550 nm and transforming into a heat-resistant polymeric material.
Heretofore, making the most of their heat-resistant property and electrical property, heat-resistant polymeric materials, typical of which are polyimides, are widely used as structural materials, printed circuit boards, heat-resistant insulating materials in the fields of electric, automobile, aeronautical, aerospace or nuclear power engineering.
On the other hand, photosensitive polymeric materials are widely used as coating materials and printing plates. Especially in recent years, making the most of their lithographic property, they have made a conspicuous achievement as fine processing materials, including resists for making printed circuits and semiconductor elements and resists for metal plate etching. Recently, extensive studies have been made to develop polymeric materials having these two useful functions, i.e., heat-resistance and photosensitivity for use as electronic or optical components. Such components include surface-protecting films, such as junction coatings, passivation films, buffer coatings and .alpha.-particle protecting films; insulation films for semiconductor devices, such as insulation films between layers for multilayer circuits; molecular-oriented films for liquid crystal display devices; and insulation films for thin-film magnetic heads or for multilayer printed circuit boards [see, for example, Functional Materials, July, pages 9-19 (1983) and Photographic Science and Engineering, pages 303-309 (1979)].
2. DESCRIPTION OF THE PRIOR ART
Heretofore, as the heat-resistant photosensitive compositions are known compositions capable of forming a crosslinked structure by actinic irradiation which comprise a polymer prepared by introducing an active functional group, such as a double bond-containing group, onto the ester side chains of a polyamic acid which is a polyimide precursor, a photosensitizer and a polymerizable monomer (U.S. Pat. No. 3,957,512 and U.S. Pat. No. Re. 30186). These compositions are basic compositions for heat-resistant polymeric materials for lithography, typical of which are so-called photosensitive polyimides. However, they are low in photosensitivity and not sufficient for practical purposes. As a composition which improves this defect, a composition comprising a polyamic acid (whose active functional group at its side chains is a methacrylate group or an acrylate group) and an azide compound, such as p-acetamidephenylsulfoneazide, as the photopolymerization initiator is proposed (U.S. Pat. No. 4,292,398). Although the photosensitivity of this composition is improved to some extent, it is still insufficient. On the other hand, in order to increase photosensitivity, compositions comprising an ethylenically unsaturated compound and an oxime ester compound (U.S. Pat. No. 3,558,309) or comprising a polyimide precursor and an oxime ester compound as the photopolymerization initiator (European Pat. No. 025230A) are proposed. These compositions, however, have either no or low photosensitivity to the g-line (wavelength : 436 nm) of a super high pressure mercury arc lamp. Also photosensitive compositions comprising a coumarin compound having a maximum absorption to a certain wave length (U.S. Pat. Nos. 4,147,552), but these compositions have some photosensitivity to the g-line which is still insufficient for practical purposes. In addition, photosensitive compositions comprising, as the main component, a mixture of a polyamic acid and an amine compound having an active functional group, such as a double bond, are proposed. However, they are solutions whose viscosity is very high, such solutions must thus be employed at low concentrations. When a spin coater and the like, which are widely employed for preparing a film on the surface of semiconductor elements, are employed, it is difficult to form a thick film from them. Further, when these compositions are left to stand after coating and drying, cracks are disadvantageously easily formed by moisture absorption due to the component capable of forming an ion bond contained therein.
Typically, the photosensitive compositions disclosed in the above described prior art are coated as solutions on a substrate, dried, exposed to actinic light, such as ultra violet rays, through a photomask and developed with a suitable developing solvent to remove unexposed portions by dissolution. Then the images thus obtained are subjected to the treatment at high temperatures to be transformed into a heat-resistant structure, such as an imide structure, and simultaneously to vaporize the volatile components, such as the side chain of the polyamic acid, the crosslinking chain and the initiator, resulting in a heat-resistant film of the images. According to conventional technology, however, when such photosensitive compositions are exposed to actinic light by using a stepper of g-line (wavelength : 436 nm) of a super high pressure mercury arc lamp which is an exposure device most popularly employed in a step of the production of LSI devices, the photosensitivity of the compositions is nil or very low and the exposure time required for hardening becomes long. Thus such compositions may be said to have a great disadvantage in view of the efficient application of such an expensive exposure device and the throughput of products.
Further, the combination of an oxime compound having a specific structure and a photosensitizer having absorption in the region of specific wave lengths is found to increase the photosensitivity of the composition by the inventors of the present invention. This combination, however, has no or insufficient photosensitivity to an actinic light of a wave length of 400 to 500 nm.